The present invention relates generally to devices for modulating intense relativisitic electron beams, and more particularly, to a device for electronically tuning the frequency of the modulation on an intense relativistic electron beam.
A new mechanism capable of modulating an intense relativistic electron beam (IREB) was recently disclosed in the article by M. Friedman, V. Serlin, A. Drobot, and Larry Seftor, Physical Review Letters 50, 1922 (1983). This IREB modulating mechanism can be used to generate a train of high-power electrical pulses (see M. Friedman and V. Serlin, Review of Scientific Instruments 54, 1764 (1983)) and to generate a pulse of high-power rf radiation (see M. Friedman, Applied Physics Letters 26, 366 (1975)). This new mechanism of modulation resembles the classical reflex klystron mechanism in that a reflexing motion of an electron beam within a drift tube produces a circulating current. However, this new mechanism does not have any form of electronic tunability of the frequency of the output current for the electron beam. Such electronic tunability is defined as the ability to change the operating frequency by varying certain electrical parameters. In the case of a reflex klystron, which operates on low density-low voltage electron beams with corresponding low-power levels, the electronic tunability is accomplished merely by changing the voltage on a reflector in the device. However, this principle for electronic tuning used in the reflex klystron is not practical for modulating IREBs, since voltages in the megavolt range would be required to reflex the electrons at this power level. At present, the only available means for tuning the IREB modulation is mechanically by changing the geometry of the modulating structure. In this regard, the output frequency from the modulating structure depends, among other things, on the transit time of an electron between components of the device. This transit time can be changed either by varying the length of the electron trajectory, or by varying the electron velocity. Previously, the output frequency was tuned by the former method, which involved mechanically moving the modulating cavities. Alternately, the frequency could be varied by replacing the modulating cavities by cavities of a different size.